Drop detectors have various applications. U.S. Pat. No. 4,583,975, Indirect Piezoelectric Drop Counter and Method, discloses a drop detector used in medical applications. U.S. Pat. No. 4,401,909, Grain Sensor Using A Piezoelectric Element, discloses a drop detector for detecting grain particles dropping and impacting a tractor's sounding board. Ink jet printers may use drop detectors to measure the presence and speed of ink drops.
The prior-art drop detectors identify drops using various approaches. One prior-art electrostatic drop detector charges a drop when the drop forms. Later, an electric-field-sensing device senses the electric field of the charged drop and produces an output signal in response to the detection of that drop. Another prior-art electrostatic drop detector detects charged drops with an electrode. When a charged drop impacts the electrode, the charge on the drop produces a small current in the electrode that indicates the presence of a drop. Unfortunately, electrostatic drop detectors have low sensitivity which makes them useless in many applications. Another prior-art drop detector directs a collimated beam of light at a photodetector. When the drops travel through the light beam, the photodetector output varies accordingly to indicate the detection of a drop. However, the collimated beam of light must be precisely aligned with the photodetector and the drop trajectory. This precise alignment is difficult and expensive to obtain and makes optical drop detectors unsuitable for many applications.
Other prior-art drop detectors are disclosed in the following patents. U.S. Pat. No. 4,583,975, Indirect Piezoelectric Drop Counter and Method, mentioned in a previous paragraph, mounts a polymeric piezoelectric film on the wall of the chamber. Instead of striking the piezoelectric film directly, the drops strike the surface of accumulated fluid and pressure waves travel through the walls of the chamber to the piezoelectric film. U.S. Pat. No. 4,401,909, Grain Sensor Using A Piezoelectric Element, mentioned in a previous paragraph, mounts a piezoelectric transducer on a sounding board. When grain strikes the sounding board, the piezoelectric transducer is stressed and produces an output signal. U.S. Pat. No. 4,128,841, Droplet Microphone, discloses a piezoelectric transducer mounted behind flexible diaphragm placed in the path of the moving droplet. U.S. Pat. No. 4,286,274, Ink Droplet Catcher Assembly, discloses a modified version of the device disclosed in the U.S. Pat. No. 4,128,841. This modified version does not have the diaphragm and exposes the piezoelectric transducer directly to the ink drops. U.S. Pat. No. 4,067,019, Impact Position Transducer For Ink Jet, discloses a relatively inflexible, piezoelectric substrate attached to two parallel, closely-spaced conductors placed in the path of a moving droplet. These prior-art drop detectors have low sensitivity that impairs their ability to detect small drops. Additionally, these prior-art drop detectors do not work reliably in environments with electrical, acoustical, or mechanical noise because their output has a low signal-to-noise ratio. Many applications need drop detectors that can detect small drops and drops traveling in rapid succession of one another. Other applications require drop detectors that can detect drops in an electrically or acoustically noisy environment. None of these prior-art drop detectors appears to perform well in such environments or applications.